The invention relates to a process and an apparatus for cutting or welding a workpiece.
Oxyfuel cutting, plasma cutting, and laser cutting are three principal methods used to thermally cut a metallic workpiece. Oxyfuel cutting is mainly used to cut mild steel where the benefits of the exothermic burning reaction of oxygen and iron are used to do the cutting. In this process, the reaction rate and the resulting cutting rate is determined by the diffusion rates of the reactants and the shear of the gas jet on the liquid metal to remove it from the cut. For cutting a mild steel workpiece having a thickness in the range from about 10 mm to about 12 mm, typical cutting speeds range from about 0.5 to about 1.5 meters/minute. Kerf widths vary from about 1 mm to greater than about 3 mm.
In plasma cutting, the energy used to cut a workpiece is supplied by an electric-arc-heated plasma gas jet which is directed toward or brought in contact with the workpiece. The plasma cutting technique works on all types of electrically-conductive materials and, therefore, has a wider application range than oxy-fuel cutting. Typical plasma arc temperatures are greater than 6000xc2x0 C. During plasma cutting, metal from the workpiece is removed from the kerf by the shear of the very high velocity plasma-arc jet. Typical cutting speeds for plasma cutting are greater than those of oxyfuel cutting. A typical cutting speed for cutting xc2xdxe2x80x3 mild steel with oxy-fuel is about 16 inches/min; whereas a 200 Amp plasma system would typically cut that same size material at 80 inches/min. Kerf widths for plasma cutting are about the same size or larger then those for oxyfuel cutting. The relatively large kerf width has an adverse influence on the precision of the plasma cutting process.
In laser cutting, the energy used to cut a workpiece is supplied by a laser beam directed toward or brought in contact with the workpiece. Material is removed from the kerf by the shear from an assist gas jet directed into the kerf. In laser cutting, kerf widths are narrow. Kerf widths typically range from about 0.15 mm to about 0.5 mm. These narrow kerf widths consequently yield higher precision cutting than is possible with either oxyfuel or plasma cutting. However, in laser cutting, it becomes difficult to remove the molten metal from the kerf as the workpiece thickness increases. This limits the cutting speed and the maximum thickness capability for laser cutting. It is believed that the reason for this limitation is that the high gas velocity required to achieve sufficient gas shear creates supersonic shock waves a few millimeters into the kerf. These shock waves limit the gas shear and its ability to remove metal.
A fourth method for thermally cutting a workpiece is disclosed in U.S. Pat. No. 5,288,960. In this thermal-cutting method, a high temperature liquid metal stream is directed at and impinges on the workpiece. The temperature of the stream exceeds the melting temperature of the workpiece. The problem of removing the molten metal from the kerf because of limited gas shear encountered in laser cutting is thus eased by using a medium (i.e., liquid) with a higher specific density. Compared to laser cutting, higher cutting speeds, thicker workpiece capability, and equivalent high precision cuts can be realized with this liquid-metal-stream cutting approach. However, because of the need to supply a high speed liquid stream to the workpiece, at a temperature greater than the workpiece melting point, this approach has been limited in its use for cutting certain metal. The material requirements for a high temperature, high pressure, liquid containment vessel severely limits the practicality of cutting metals such as aluminum, stainless steel and mild steel, where typical melting temperatures are 660xc2x0 C., 1400xc2x0 C. and 1550xc2x0 C., respectively.
Several methods are used to thermally weld a workpiece. The most widely used welding processes use heat sources to cause localized heating of two or more workpieces, allowing them to melt and flow together. A filler metal generally is added to the weld area in order to supply sufficient material to fill the joint and to increase mechanical strength. For example, a fillet weld generally forms a radial sector of additional material over a weld groove when completed. When the welding process is progressing, a molten pool of workpiece forms and a filler material is moved along the welding front. When the welding heat source is removed, the molten metal solidifies, and the parts are fused or welded together. Common heat sources used to provide heat to melt the workpieces are DC or AC electrical arc, oxy-fuel gas flame, and laser beam.
An objective of this invention is to provide a very high energy density fluid stream which can be used in materials working processes. Another objective of this invention is to provide a process and an apparatus for thermally cutting workpieces at high speed and high precision over a large range of workpiece thicknesses. Another objective of this invention is to provide a process and an apparatus for thermally welding workpieces at high speed and high precision. Another objective of this invention is to thermally cut and/or weld non-metallic and/or non-conducting materials. A further objective of this invention is to provide a process and an apparatus of cutting and/or welding which is simple in design, easy to operate and maintain and cost effective to use.
In one aspect, the invention features a system for modifying a workpiece. The system comprises a dispenser and a power source. The dispenser comprises an electrically conductive material for forming a jet stream. The power source is electrically coupled to the jet stream.
In one embodiment, the dispenser comprises a jetting head. For example, the jetting head can comprise a crucible. A heater can be coupled to the crucible. The heater can comprise one of an AC resistance heater, a DC resistance heater, an induction heater, or a combustion burner-heater arrangement. The heater can comprise an induction heater coil wrapped around the crucible. In one example, the induction heater coil wrapped around a first end of the crucible has a closer packed relationship than the induction coil wrapped around a second end of the crucible. In another example, the induction heater coil wrapped around a first end of the crucible has a smaller diameter than the induction coil wrapped around a second end of the crucible. The system can further comprise a depressurizing vent in communication with the pressure containment vessel. The crucible can comprise a refractory material. For example, the crucible can comprise a material selected from one of zirconium diboride, alumina, zirconia, boron nitride, and graphite. The conductive material for forming the jet stream can comprise a metal.
The jetting head can comprise an inlet for receiving a feed stock of the conductive material. In another embodiment the jetting head can comprise multiple inlets for receiving multiple feed stocks of conductive material. The jetting head can further comprise a feed stock valve. The jetting head can comprise a pressure containment vessel and a heater disposed inside the pressure containment vessel. The system can further comprise a pressurizing gas source in communication with the pressure containment vessel. The jetting head can comprise an electrode disposed inside the crucible for establishing an electrical connection with the jet stream.
The jetting head can comprise an exit orifice. In addition, the jetting head can further comprise a plug. In this embodiment, the jetting head can comprise a plug rod disposed above the exit orifice. The jetting head can further comprise a nozzle. The nozzle can comprise a disk having a conical opening. The jetting head can further comprise a nozzle and a nozzle cap detachably attached to the pressure containment vessel adjacent the nozzle. In one embodiment a filter can be placed in series with the nozzle. In another embodiment the crucible has a conductive fluid filter.
In one embodiment, the system for modifying a workpiece further comprises a first lead electrically coupled to the power supply and a work piece and a second lead electrically coupled to the power supply and a conductive fluid disposed in the crucible. In another embodiment, the system of can further comprise a first lead electrically coupled to the power supply and a work piece clamp and a second lead electrically coupled to the power supply and a conductive fluid disposed in the crucible. In still another embodiment, the system can further comprise a first lead electrically coupled to the power supply and a current collector. For example, the current collector can comprise a vessel.
In still another embodiment, the system can further comprises a first lead electrically coupled to a first power supply and a first feedstock and a second lead electrically coupled to the first power supply and a second feedstock. The first and second feedstocks making electrical contact with the conductive fluid disposed in the crucible. The two feedstocks are heated by passing current between them. A second power supply comprises a first lead electrically coupled to the work piece and a second lead electrically coupled to the power supply and a feedstock of the first power supply.
The jetting head can further comprise a shield assembly supporting the nozzle. The shield assembly can comprise a disk having a plurality of inlet orifices for introducing a shield gas to the jet stream.
In another aspect, the invention features a metal jet cutting system. The system comprises a jetting head including an exit orifice for dispensing a jet stream of a conductive fluid and a power source electrically coupled to the jet stream for providing a current to the jet stream to increase a temperature of the jet stream above a melting temperature of the conductive fluid.
In still another aspect, the invention features a process for modifying a workpiece. According to the process, a jet stream comprising a conductive fluid is provided. An electrical current is passed through the jet stream. The jet stream is directed at the workpiece for modifying the workpiece.
The jet stream can be heated in a variety of ways. A current can be applied to the jet stream through an electrode coupled to the conductive fluid and a current collector disposed near the workpiece. A current can be applied to the jet stream through an electrode coupled to the conductive fluid and a workpiece clamp. The jet stream can be heated through ohmic power dissipation. The jet stream can be heated to a temperature substantially above a melting temperature of the conductive fluid. A temperature of the jet stream can be increased up to about 1000xc2x0 C. above a melting temperature of the conductive fluid. The jet steam can be a continuous jet stream, a pulsed jet stream, a steady jet stream, or an unsteady jet stream.
In one embodiment the heater of the crucible is an induction heater where the characteristic frequency of the induction heater can be calibrated to the level of a conductive fluid in the curcible.
In one embodiment, the feed stock and the workpiece comprise the same type of material. Alternatively, the feed stock can the workpiece can comprise different types materials. For example, the feed stock can comprise aluminum and the workpiece can comprise stainless steel. The feed stock can be a conductive fluid. Alternatively, the feedstock can be heated to form a conductive fluid. In one example, the feed stock is a metal such as aluminum, iron, an iron containing compound, tin, nickel, titanium, gold, platinum, silver, magnesium, copper, mild steel or aluminum-iron alloy. The feed stock can comprise a wire, bar, or powder. In still another embodiment the feedstock can comprise a wire or bar and also serve as an electrical contact between a power source and the conductive fluid. More than one feed stock can be in contact with an electrical power source. The feed stock can comprise a plurality of non-melting particles. The non-melting particles can be abrasive. The feed stock can have a low melting point and a high boiling point.
The exit orifice of the crucible can be plugged while providing the feed stock and the exit orifice can be unplugged while the conductive fluid passes through the exit orifice. A vacuum can be provided to the jetting head to plug the exit orifice. A levitation force can be provided to the conductive fluid to plug the exit orifice.
In one embodiment, the jetting head is pressurized while passing the conductive fluid through the exit orifice. For example, the jetting head can be pressurized by supplying an inert gas.
In another aspect, the invention features a crucible for a metal jet cutting system. The crucible comprises side walls and a base. The crucible is electrically conductive and is resistant to dissolving in the presence of a metallic melt. The crucible can be formed from a zirconium containing compound. The crucible can also be formed from zirconia diboride or yitria-stabilized-zirconia.
In another aspect, the invention features a nozzle for a metal jet cutting system. The nozzle comprises a disk-structure having an orifice, wherein the orifice is located at a center of the disk-structure. The nozzle is electrically conductive and is resistant to dissolving in the presence of a metallic melt. The nozzle can be formed from a zirconium containing compound. The nozzle can also be formed from zirconium diboride.
Various parameters can be controlled when the process of the present invention is performed. For example, a pressure in the jetting head, a temperature of the conductive fluid, a depth of penetration of the jet stream and/or a velocity of the jet stream can be controlled.
In one embodiment, the workpiece can be cut, marked or pierced. Alternatively, the workpiece can be welded. For example in welding, a first workpiece having a first tapered edge and a second workpiece having a second tapered edge are provided. The first tapered edge is positioned adjacent the second tapered edge to provide a groove. The jet stream is directed at the groove to fill the groove. Directing the jet stream at the groove can melt a portion of the workpiece forming a molten pool in the groove. Cooling the molten pool welds the first workpiece and the second workpiece.
In one embodiment, a workpiece can be modified by lowering a melting point of the workpiece. The melting point can be lowered by forming an alloy of the feed stock material and the workpiece material on a surface of a portion of the workpiece. The process of modifying a workpiece can further include providing a shielding gas to shield the jet stream.
In one embodiment, the process of modifying the workpiece can be used to modify an insulative material. When modifying an insulative material, a current collector comprising a conductive material can be disposed underneath the workpiece. The current collector forms an electrical contact with the jet stream.